This invention relates generally to gas turbine engines and more particularly to a multi-spool by-pass turbofan engine having specific utility as the power plant of a jet aircraft.
Multi-spool by-pass turbofan engines have been accepted by the aircraft industry as efficient and dependable power plants. A typical engine comprises a low pressure spool having a fan at a forward end and a low pressure turbine at the aft end thereof. A high pressure spool has a high pressure compressor in fluid flow communication with the fan on the low pressure spool and a high pressure turbine in fluid flow communication with the low pressure turbine on said low pressure spool. In certain applications, an intermediate pressure compressor is orientated downstream of the fan, either as part of an intermediate pressure spool or as a component of the low pressure spool. If mounted on a separate intermediate pressure spool, the intermediate pressure compressor is coupled to an intermediate pressure turbine.
The fan on the low pressure spool is generally aligned with an annular by-pass duct that surrounds the high and low pressure spools.
A combustor is located between the high pressure compressor and high pressure turbine of the engine. Engine accessories including a generator and hydraulic pump are driven by the high pressure spool. The aforesaid geometry is relatively efficient and is well proven in field use.
One requirement of an aircraft jet engine is that the time required to accelerate the high pressure spool of the engine from idle RPM to full rated RPM be minimized so as to minimize the time necessary to achieve full rated thrust for airspeed recovery in the event of a stall or aborted landing approach. The engine acceleration time frame is generally minimized by utilizing a relatively high idle RPM for the high pressure spool of the engine.
Another factor to be considered is that an aircraft requires substantially more electrical power on the ramp than is available from the aircraft's battery. On large aircraft, electrical power is generally supplied by an onboard auxiliary power unit. On smaller aircraft, such as business or commuter aircraft, the cost, weight, and cube exhibited by an onboard auxiliary power unit are prohibitive. Accordingly, electrical power, hydraulic power and air cycle air conditioning are often supplied by the aircraft's engines. In order to produce the necessary power, the high pressure spool of the engine must exhibit a relatively high idle RPM.
An anomaly exists, however, in satisfying the aforesaid requirements because the high idle RPM of the high pressure spool necessary to minimize the engine acceleration time frame from idle to full RPM and to supply adequate power on the ramp characteristically increases idle thrust to an unsatisfactory level. High idle thrust compromises both the landing maneuver and ground operation of the aircraft.